Rigs are deforming strangely
The lower arm mesh is influenced by the upper arm bone. You need to do several things:


First put your Armature modifier above the Subsurf, enable Preserve Volume, plus I don't think you need 2 for Viewport Subdivisions but it's a detail.
Your topology needs to be aligned on your bones and also you need to make sure that it will allow your elbow to bend smoothly, so move your elbow edge and bevel it.
Select the armature then the object, switch to Weight Paint mode, in the N panel > Tool, choose Blend > Subtract or Mix depending on if you want to subtract or add influence, in the Options only keep 2D Falloff so that you'll be able to paint through the mesh.
Select the upper arm and paint its influence so that it doesn't cross the elbow, same for the lower arm.
Move your bones to test.